1. Field of the Invention
This invention relates to an auto scan horizontal deflection apparatus cable of obtaining constant, horizontal amplitude even when the horizontal deflection frequency is changed.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional auto scan horizontal deflection apparatus disclosed, for example, in Japanese Utility Model Application Laid-Open No. 63-56865 (1988). In the figure, reference numeral 1 designates a horizontal oscillation driving circuit to which a horizontal synchronizing signal is inputted from an exterior image apparatus through an input terminal IT, and numeral 2 designates a horizontal output circuit (driven by the output of the horizontal oscillation driving circuit 1.
In the horizontal output circuit 2, the symbol Q designates a switching transistor which is switched by the output of the horizontal oscillation driving circuit 1, and D1 designates a damper diode thereof. The symbol L designates a horizontal deflection coil, and C designates a resonance capacitor connected with the horizontal deflection coil L in parallel.
In addition, a plurality of dummy coils Ld, each of which can be short-circuited by a switch S1, are connected in series with the horizontal deflection coil L, and a plurality of dummy capacitors Cd, each of which is grounded at its one end through a switch S2, are connected in parallel with the resonance capacitor C.
Symbol Cs designates an S-form compensation capacitor connected in series with the horizontal deflection coil L, and R designates a detection resistance connected in series with the S-form compensation capacitor Cs in order to detect horizontal deflection current I flowing through the horizontal deflection coil L.
Reference numeral 3 designates a peak rectifier circuit which detects voltage drop between both of the terminals of across the detection resistance R, and 4 designates a variable power circuit in which a source voltage Vcc to be outputted is controlled with the output voltage of the peak rectifier circuit 3 being used as control signal.
Reference numeral 5 is a choke transformer supplying the source voltage Vcc to the horizontal output circuit 2 through a primary winding 5a from the variable power circuit 4, and a high-voltage output is outputted from a secondary winding 5b thereof through a diode Db.
Reference numeral 6 is a deflection mode switching circuit controlling opening/closing of the switches S1 and S2 inside of the horizontal output circuit 2, responsive to the frequency of title inputted horizontal synchronizing signal
Next, an explanation will be given of the operation of the circuit shown in FIG. 1. A plurality of dummy windings Ld which can optionally be short-circuited by the switch S1 are connected in series with the horizontal deflection coil L so that the power voltage Vcc of the horizontal output circuit 2 is not changed greatly when the frequency of the inputted horizontal synchronizing signal is changed.
In addition, at the resonance capacitor C, a plurality of dummy capacitors Cd are connected in parallel, each of whose one end being grounded respectively by the switch S2 so that the ratio Ts/Tr of the horizontal scanning time Ts to the horizontal flyback time Tr may be kept almost constant by changing the horizontal flyback time Tr.
These switches S1, S2 are switched according to frequencies of the inputted horizontal synchronizing signals of a television system (15 to 17 KHz), high density personal computer system (22 to 26 KHz), high density CAPTAIN (Character and pattern telephone access information network:videotex) system (29 to 34 KHz) and so on by the deflection mode switching circuit 6.
By the way, of background horizontal width of an image projected on a picture tube is decided by the horizontal deflection current I when the anode voltage of the picture tube is constant. The relation between the horizontal deflection current I and the source voltage Vcc of the horizontal output circuit 2 is shown in the following equation (1). ##EQU1##
Accordingly, variation of the horizontal width can be limited to a certain extent by changing the net inductance L+Ld including the horizontal deflection coil L at the horizontal scanning time Ts is changed with the horizontal deflection frequency. In order to make the horizontal width constant usually, however, it is required to control the source voltage Vcc by detecting the horizontal deflection current I.
Therefore, the detection resistance R is connected with the S-form compensation capacitor Cs and the voltage drop across this detection resistance R is detected by the peak rectifier circuit 3. The variable voltage circuit 4 is then controlled so that the value of the detected voltage drop may be constant, thereby adjusting the source voltage Vcc outputted therefrom.
In addition, as another prior art, the invention of Japanese Patent Application Laid-Open No. 62-5781 (1987), entitled as "a deflection circuit" has been proposed. The invention of this Japanese Patent Application Laid-Open No. 62-5781 (1987) is so constructed as to carry out almost a constant horizontal deflection by changing the electric source potential of the horizontal output circuit to be saw toothlike responsive to the input frequency of the horizontal synchronizing signal.
As a conventional auto scan horizontal deflection apparatus is so constructed as above-mentioned, in the former, electric power is largely lost according to the detection resistance R, and the deflection current I becomes large especially when a large-sized monitor is used, therefore electric power loss becomes great and cannot be neglected, these facts causing the apparatus to be an impractical one.
The latter arrangement has a possibility that the horizontal scan width changes when the input frequency of the horizontal synchronizing signal changes. Because it does not carry out feedback of the horizontal scan width.